This invention relates to a method and device for implementing the said method, designed to separate, in a gaseous mixture, at least one isotopic compound from another or others contained in said mixture.
More particularly, the invention is aimed at separating stable isotopes of the same body, in order to isolate one of them for its properties and characteristics that make it attractive for the applications and uses envisaged.
There are several methods of separation, most of which are operational, based firstly on the conversion into gaseous form of the body, and secondly on the difference in atomic weights of the isotopes.
The invention relates to methods wherein the gas, possibly after mixing with another gas, namely an inert gas such as helium, argon or nitrogen, flows through a device designed to create a gaseous flow at supersonic speed which is subjected to means of excitation and chemical or physical modification such as lasers or electron beams that have a greater effect on the isotope sought than on the other or others. Then, the isotopes thus modified in gaseous form are, after ionisation, separated by electromagnetic means that make one of the isotopes migrate to the outside of the gaseous flow resulting in the enrichment of the sought isotope.
In general, the gaseous flow is generated by a nozzle, such as a so-called de Laval nozzle, for example, and the excitation and modification means are lasers or electron beams.
For example, the gaseous compound may be SF6, UF6, or WF6, MoF6. In the case of uranium the isotope sought is then U235; in the case of sulphur, the isotope sought is S34 and in the case of molybdenum, the isotope sought is Mo99.
Thus, the aim of these methods is to enrich with the sought isotope the external or internal part of the gaseous flow after separation.
The methods and devices for isotope separation need to reconcile contradictory constraints.
That is because they must simultaneously:                Consume as little power as possible        Be as small as possible        Make it possible to achieve enrichment rates (content of the isotope sought) that are as high as possible        Use quantities of gas, thus the starting body, that are as low as possible according to the final quantity of enriched product sought at a given enrichment rate.        
The known methods and devices are still very far from optimising the work required for separating isotopic species. As a result of the low enrichment rates achieved in a single pass of gas in common devices such as centrifuges, the installations must be designed in a serial manner to obtain acceptable enrichment rates. That means that the installations have to be large, with high power consumption in relation to the minimum thermodynamic work required for enrichment. The patent FR 2 370 506 describes an example of such installations.
Other methods or devices, such as that described in the U.S. Pat. No. 4,119,509, lead to a flow regime that is not compatible with the efficient separation of isotopes. That is because the device does not inhibit diffusion, resulting in a mixture of isotopologues in the gaseous flows, greatly limiting the efficiency of the method.
The invention discloses a method and device that constitute a significant advance in the relevant field by allowing high-efficiency isotope separation with reasonable quantities of gas and power, using a small installation.
To that end, according to the invention, the method for the isotopic separation of at least two different isotopes of a body in gaseous form comprises the steps stated in claim 1.
The ion recovery step is preferably carried out with ion recombination.
The gas may, for example, be SF6, UF6 or MoF6.
The molecules are excited by vibrations with the help of at least one laser, for example of the CO2 type in the case of UF6 gas.
The gas is mixed with at least one first inert gas, such as helium or argon.
The speed of the gaseous flow is supersonic.
Advantageously, the means to create the laminar or slightly turbulent flow is a nozzle, of the de Laval type, preferably with flat geometry comprising:                upstream walls that form a convergent;        a throat with a parallelepiped section at the exit of the upstream convergent;        a divergent conduit abutted to the throat, with a so-called exit section, wherein the exit corresponds to the flared part of the divergent conduit.        
The nozzle is supplied with a gaseous flow that firstly comprises a central or inner jet formed of a mixture of gas to process and a first other gas, namely an inert gas, and secondly a peripheral or outer jet formed of a second other gas, namely an inert gas.
The two jets are substantially isolated from each other. In the case of a revolution nozzle, the two jets are concentric.
The first and second inert gases may be the same, and are selected from helium, nitrogen, argon and hydrogen.
The nozzle is supplied by means of a chamber supplied with gas, which has:                an exit wall corresponding to the upstream convergent of the nozzle        an internal deflector with convergent walls, substantially concentric with said upstream convergent of the nozzle, and at a distance from it.        
The internal deflector is located upstream from the throat of the nozzle or extends downstream up to it.
A mixture of said gas and a first inert gas flows in the inner part of the deflector and a flow of a second inert gas is provided between the deflector and the convergent of the nozzle. These two gases are independent and come from two separate tanks.
After expansion in the divergent of the nozzle, part of the isotopes present are made to migrate from the inner flow to the outer flow.
The nozzle is generally planar in shape, so as to increase the efficiency of the process by minimising the mass flow of the species flowing in the boundary layers and maximising the mass flow of the species flowing in the isentropic core, where selective laser radiation absorption occurs.
The nozzle may for example have the following dimensions:                Curvature radius of the convergent of the nozzle: 260 mm        Curvature radius of the internal deflector: 250 mm        Height of throat: 100 mm        Length of downstream divergent 2000 to 3000 mm        Width of the exit section of the divergent: 1200 mm        
The exit section of the internal deflector is positioned near the throat of the nozzle or offset by a few millimeters upstream from said throat.
These dimensions correspond to a speed of Mach 4, which itself corresponds to a temperature of 40° K. at the core of the flow. That allows satisfactory selectivity of laser radiation absorption and thus guarantees a high enrichment rate.
The width of the exit section of the conduit ranges between 5 and 20 times the width of the throat, and is preferably close to 7 times the width of the throat.
The length of the divergent conduit is at least 2000 mm and/or ranges between 20 and 30 times the height of the throat.
In the case of a revolution shape, the nozzle has the following dimensions as an example when it is of the revolution type:                Deflector curvature diameter: 250 mm        Curvature diameter of the internal deflector: 250 mm        Diameter of throat: 100 mm        Length of downstream divergent 1680 mm        Diameter of the exit section of the conduit: 340 mm        
The diameter of the exit section of the conduit ranges between 3 and 6 times the diameter of the throat, and is preferably between 3 and 4 times
The length of the divergent conduit is at least 1300 mm and/or ranges between 13 and 30 times the diameter of the throat.
Whether the nozzle is planar or of the revolution type, the diffusion speed Vd is far smaller than the average speed Vq in the section in question of the nozzle Vd=D/e, where D is the diffusion coefficient and e is the distance that separates the walls opposite the external and internal deflectors.
Means to inhibit or reduce the phenomena of nucleation of molecules of the gaseous flow are provided in the conduit or in the flow outside the nozzle, and preferably, said means comprise at least one laser, for example of the infrared type, where the light direction is transversal to the gas flow.
Cooling means may be used in order to reduce the viscosity of the gas at the walls and thus the flow of the carrier inert gas. These cooling means allow gas flow temperatures between 4° K. and 80° K., preferably between 4° K. and 10° K.
The method is based on the uniformity of the flow at its core, which means that the pressure and temperature conditions remain unchanged in the direction of the flow over lengths of a few meters and transversally over several tens of centimeters. The shapes of the nozzles described make it possible to validate these conditions.
The invention will be better understood in the light of the description below relating to illustrative but non-limitative examples, by reference to the figures, wherein: